This invention relates to an improvement in the noble metal-containing overglaze color composition in a liquid or paste form for application to ceramics.
So far, Rh (rhodium) has been used as a main component of the sintering inhibitor for noble metalcontaining overglaze color compositions in a liquid or paste form to be applied to ceramics, inclusive of pottery, chinaware, glassware, etc., with Cr, Bi, V and/or B, for instance, being additionally used, so that a pretty noble metal mirror surface can be obtained on the ceramic surface by firing and the alkali resistance of the thin noble metal film or layer produced by firing can be improved.
However, since the demand for rhodium is great in spite of limited sources therefor, the price of rhodium is increasing and it is becoming more and more difficult to use rhodium as a sintering inhibitor for noble metal-containing overglaze compositions.
Furthermore, when rhodium is used, the alkali resistance of the noble metal mirror surface formed on the ceramic surface is not fully satisfactory. For instance, in a boiling alkali resistance test at 100.degree. C. using a 2% aqueous solution of sodium carbonate, the noble metal (gold) mirror surface layer formed from the noble metal- and rhodium-containing overglaze composition specified later herein in Table C-I completely peeled off after the lapse of about 10 minutes in the case of firing at 600.degree. C., about 50 minutes in the case of firing at 700.degree. C., and about 80 minutes in the case of firing at 800.degree. C.
Thus, the conventional use of rhodium as a sintering inhibitor for noble metal-containing overglaze compositions encounters not only the problem that it is very costly but also the problem that it does not always lead to satisfactory alkali resistance.